[Earlofessex]

I've read about Efficient Dynamics and the fact that my F25 is smart enough to only engage the alternator when the battery needs charging. I've also noticed that when I put the climate control in Auto mode then the AC snowflake light is always on. Does this mean that the AC compressor is always engaged when in Auto mode, e.g., even when it is below freezing outside? Or does the Efficient Dynamics approach somehow also apply to the AC?

[bren]

Nothing to do with the efficiency business, that's just the way the auto climate works.

[Earlofessex]

So snowflake light on means compressor engaged and impacting gas mileage, so don't press the Auto button in winter unless the windows are fogging. Sound right?

[Borns]

In winter AC ON means that DRY air is circulating so that the windows don't get foggy...
However, I am wondering myself if this process requires more energy...

[Earlofessex]

Yeah, that's where my question is coming from. Drying the air means "conditioning" it with the AC and then heating it with the heater, so I'm thinking snowflake light on means a hit on mileage.

[Earlofessex]

How can the compressor run very little yet still continuously dehumidify? Does the climate control system have a humidistat? If not, it would seem that the defogging would be intermittent.

[n55x3]

On all our vehicles with auto climate control we have always disengaged the A/C compressor via the 'snowflake' button during the colder winter months to avoid unneeded parasitic loss. We only activate the A/C compressor when we need to defrost the inside windows.

And as far as I know you are only dehumidifying when the compressor is engaged and actively running. When the compressor is out of the loop you are simply moving air around.

[RhoXS]

Originally Posted by Lotus7

When to "snowflake" light is on, the A/C compressor MAY be engaged as needed to cool, defog or defrost the interior. In cold weather, the compressor will run very little, so should not have any significant effect on fuel consumption, at least not much when compared with the increased fuel consumption during warm-up due to the engine and drive-line lubricants being cold.

There is another issue related to keeping the A/C compressor off for long periods. The A/C compressor is lubricated by compressor oil that is carried in the A/C coolant gas. The compressor shaft seals are lubricated by the circulation of the oil in the gas. If the compressor is not run for very long periods of time, the shaft seals dry and shrink and allow the coolant gas to leak out. The general "rule" is that if you turn off your A/C system for the winter, it's still a good idea to force the compressor on by pressing the front windshield defrost button for 3 or 4 minutes at least once every week or two.

I agree completely. In addition:

The climate control systems in today's vehicles are refined automatic systems designed to control temperature and humidity under almost all conditions and these systems do it well. Typically, above 40 degrees F or so the compressor is used in a reheat cycle to control humidity. Below 40, just the warming of the air will lower the dew point sufficiently for a comfortable humidity and the systems are smart enough to not run the compressor (same reason a humidifier is required during winter months when heat is used).

In any case, manually turning off the compressor is only saving pennies and not allowing the system to work as designed. The X3 and similar vehicles are certainly not inexpensive. However, we manually defeat an automatic function that only might save pennies after spending a lot of money for the fine engineering behind these systems and the creature comforts these systems provide. Not sure I understand the logic.

One last thing. Universally, the snow flake light only means the system is enabled to run the compressor. It does not mean the compressor is engaged continuously while the light is on. Except under conditions of very high heat load, the compressor is always cycling to prevent the evaporator from freezing up but the light remains on continuously. BTW, the compressor is not cycled to control cabin temperature, the amount of reheat takes care of that function.

[bren]

Originally Posted by RhoXS

In any case, manually turning off the compressor is only saving pennies....

I think pennies might even be generous.

[n55x3]

I certainly don't disengage the compressor for financial gains, although every little bit helps, I keep my compressor off because when it is on I feel a slight drag in my vehicle which is normal and minor but none the less it is there. I also don't believe the automated systems are as smart as some believe. I could be wrong but I don't think our X3, or any other vehicle in our fleet, measure cabin humidity levels and are smart enough to cycle on and off when our windows get fogged over.

If the fuzzy logic behind the auto climate control was well defined then there wouldn't even be a button to turn the ac compressor on and off it would be in AUTO mode always. BMW, as well as pretty much every other car manufacturer out there, recognizes the need to permanently place the compressor in a persistent OFF mode due to many factors thus the availability of the button.

I personally like to be able to engage and disengage various accessories when not needed especially when there is a direct correlation to my vehicles performance and efficiency.

[bren]

Maybe you should remove the pulley from the compressor and install a shorter belt in the winter.

Actually, it would probably be best to remove the entire a/c system in the winter to shed the 40 or so pounds of useless weight - imagine the performance benefits on your trips to the mall.

[n55x3]

Originally Posted by bren

Maybe you should remove the pulley from the compressor and install a shorter belt in the winter.

Actually, it would probably be best to remove the entire a/c system in the winter to shed the 40 or so pounds of useless weight - imagine the performance benefits on your trips to the mall.

See now you are thinking!! There is always more than one way to skin a cat!

Gosh, I can see it now, the windows fog over on my wife's X3, she pulls over on the highway and struggles to crank on the belt tensioner to give her enough slack to lift the shorter belt off and replace with the std length belt so the ac compressor can spin up and defog her windows. After she does that a few times I'd give her the junior mechanic label!! ;-)

In the end though you are right, whether you are mechanically eliminating the ac compressor from the serpentine loop all together (via a shorter belt) or you are simply pressing the ac compressor on/off button on the dash the end results should be the pretty much the same, no parasitic loss. Just a slightly different level of involvement to reach the same goal.

[bren]

Originally Posted by n55x3

...whether you are mechanically eliminating the ac compressor from the serpentine loop all together (via a shorter belt) or you are simply pressing the ac compressor on/off button on the dash the end results should be the pretty much the same, no parasitic loss.

I don't know - just pushing the button means you are still spinning a superfluous pulley; only the compressor clutch will never engage.

[RhoXS]

Originally Posted by n55x3

I certainly don't disengage the compressor for financial gains, although every little bit helps, I keep my compressor off because when it is on I feel a slight drag in my vehicle which is normal and minor but none the less it is there. I also don't believe the automated systems are as smart as some believe. I could be wrong but I don't think our X3, or any other vehicle in our fleet, measure cabin humidity levels and are smart enough to cycle on and off when our windows get fogged over.

...

I personally like to be able to engage and disengage various accessories when not needed especially when there is a direct correlation to my vehicles performance and efficiency.

I am not aware of any vehicle that actually measures humidity because this level of control is simply not needed and, imo, be bad and irresponsible engineering if it was used. The air passing over the cold evaporator core significantly lowers the dew point, forcing the entrained moisture to condense out. The now much drier air then passes over the heater core to be warmed sufficiently, as deemed necessary by the computer, to control the cabin to whatever is the desired temperature. There is no reason for a humidity sensor because you really cannot get the cabin too dry and precise humidity control would make the system unreasonably complex and expensive.

Each of us certainly has the right to do what we want with our vehicles. However, imo, regardless of the rationalization given, there is not a single consequential engineering reason either from an economy perspective or performance perspective to ever disable the compressor. By performance I mean both the ability of the system to control the cabin interior climate and the performance of the vehicle both in terms of efficiency, acceleration, and braking. If it just makes one feel better not seeing the snowflake lit then admit that is the reason. If vehicle performance is the issue, then only at full throttle acceleration will one see a minor difference and then, at full throttle, the gasoline being sucked up is more than negating the small economy of not running the compressor.

That being said, there is indeed sometimes a good reason for overriding an automatic function. IMO, all vehicles I have had with automatic climate control, without exception, do not do a good job deciding when to switch from recirc to outside air. On hot days they invariably come off recirc too soon. I also like it really cold with the least fan noise. Therefore, if the temp is in the high 70s F or greater, and there is a significant sun load on the car, I always force recirc. To me, it feels colder and drier when I do this. I also suspect much of this is in my head, and I paid for the car, so I do what I want, at least until my wife complains.

[Nahoa]

+1